Phrase-structure rules were used in early transformational grammar (TGG) to describe a given language's syntax. This was accomplished by attempting to break language down into its constituent parts (also known as syntactic categories) namely phrasal categories and lexical categories (aka parts of speech). Phrasal categories include the noun phrase, verb phrase, and prepositional phrase; lexical categories include noun, verb, adjective, adverb, and many others. Phrase structure rules were not an invention of TGG; rather, early TGG's defining characteristics were those systems which it had in addition to phrase structure rules (the most obvious example being transformations;see the page transformational grammar for an overview of the development of TGG.) A grammar which uses phrase structure rules is called a phrase structure grammar.
Definition
Phrase structure rules are usually of the form 
, meaning that the constituent A is separated into the two subconstituents B and C. Some examples are:
